I downloaded Shadow over dedede v4 and used wiiscrubber to replace .pcs and .pax files. Then I used swandz to insert shadowvoice.swandz into smashbrossound.bras. But when I put the modded iso into the usb and start ssbb (I use usb loader channel btw), it crashes whenever I choose shadow. Anyone who can give help with this? And is there any shadow port without slow running start glitch?

It's nearly 100% likely because of filesize; compare filesize of the original Brawl files with the ones you're replacing with; if filesize is NOT the problem, then I'll talk about the less likely possible reasons

Figure out which of the .pac and .pcs files are bigger in size than the ones from Vanilla Brawl, and for the ones that are bigger in filesize, you can export textures with large dimensions, and then Replace them with the same textures, but in the import window, change the dimensions to smaller, but make sure that it's by a power of 2